much easier solution which only involves one change in pos_payement_terminal

extra_addons/pos_payment_terminal/static/src/js/pos_payment_terminal.js

@@ -61,6 +61,22 @@ odoo.define('pos_payment_terminal.pos_payment_terminal', function (require) {
             var order = this.pos.get_order();
             var line = order.selected_paymentline;
             var data = self.get_data_send(order, line, currency_iso);
+            //Special case of meal voucher payement line :
+            if(line.is_meal_voucher && line.is_meal_voucher()) { //Make sure function exists (module meal_voucher might not be active)
+                //TODO : factor (already 3 occurences of this code in pos_meal_voucher)
+                var total_eligible = order.get_total_meal_voucher_eligible();
+                var total_received = order.get_total_meal_voucher_received();
+                var max_amount = this.pos.config.max_meal_voucher_amount;
+                var current_max = total_eligible;
+                if (max_amount) {
+                    current_max = Math.min(total_eligible, max_amount);
+                }
+                // Check how much is still possible to pay with meal voucher
+                // The selected line is "by default" set to the rest to pay of the order
+                const max_curent_amount = current_max-total_received +order.selected_paymentline.get_amount();
+                data["amount"] = round_pr(Math.max(0,max_curent_amount), this.pos.currency.rounding);
+            }
+            console.log(data["amount"]);
             if (this.wait_terminal_answer()) {
                 screen.$('.delete-button').css('display', 'none');
                 this.message('payment_terminal_transaction_start_with_return', {'payment_info' : JSON.stringify(data)}, { timeout: 240000 }).then(function (answer) {